A patent document 1 (Japanese Patent No. 5552534) discloses a flat-type buckle. In the buckle disclosed in this document, a leg (12) of a plug (10) is pivotable in the thickness direction of a socket (20) at the inside of the socket (20). An engaging portion (14) provided at the tip of the leg (12) engages with the engaged portion (24) at the socket (20). For releasing the engagement between the engaging portion (14) and the engaged portion (24), the socket (20) has a manipulation portion (27A) which is pivotable in the thickness direction of the socket (20). As described at paragraph 0019 of the same document, the leg (12) slants to be closer to the side of the manipulation portion (27A) along the insertion direction of the plug (10) into the socket (20). Accordingly, slight bending of the manipulation portion (27A) causes the leg (12) to pivot downward.